Why February birthdays break most Saju calculators

The Saju year does not begin on 1 January. It does not begin at Lunar New Year either, which is the more common wrong answer.

It begins at Ipchun — the start of spring — which falls around 4 February each year, set by the sun's actual position rather than by any calendar.

What this does to a chart

Someone born on 3 February 2024 is in the Water Rabbit year. Someone born on 5 February 2024 is in the Wood Dragon year.

They are two days apart and they are not in adjacent years of the system. They are in different years, with a different year pillar, and — because every judgement in a reading is relative — a different frame for the whole chart.

Why so many tools get it wrong

Two reasons. The lazy one is treating 1 January as the boundary, which is simply importing the wrong calendar.

The subtler one is treating Lunar New Year as the boundary. This is wrong but understandable: Korea does use a lunar calendar for holidays and traditional birthdays, so the two systems sit side by side in daily life and get conflated constantly in English writing. Lunar New Year moves between late January and late February and has no fixed relationship to Ipchun.

The same problem, smaller, every month

Month pillars turn on solar terms too — twelve of the twenty-four act as boundaries, landing a few days either side of the fourth of each month.

So "I was born in July" tells you nothing about your month pillar until you know whether you were born before or after that month's boundary. Early-in-the-month births are the fragile ones.

How to check

If you were born in the first week of February, or in the first few days of any month, find out what your chart calculator used as the boundary. If it cannot tell you which solar term decided your month pillar, assume nothing.